1. Field of the Invention
The device of this invention is a tap and more particularly relates to an improved design of a cold-forming tap.
2. Description of the Prior Art
Conventional cold-forming taps are well known in the art. They normally are circular in shape with radially ground lobes. In order to manufacture them, these lobes must be ground into hardened, high-speed steel. To manufacture conventional thread-forming taps, a cam-relieving method is used whereby the tap blank is rotated between holding centers on a thread grinder while a grinding wheel shaped to the desired thread form is plunged symetrically toward and away from the axis of the blank by a cam action. This movement results in the formation of high points or lobes running parallel to the axis of the tap which are commonly referred to as forming lobes. To manufacture such lobes requires slow grinding speeds and multiple passes to prevent overheating of the tap blank during the part of the grinding cycle that plunges the grinding wheel toward the axis of the tap. After the thread form is ground in the tap blank, a secondary operation to grind the outer diameter of the lobe to conform to the thread shape must be performed. This secondary grinding operation is done at the crest of the thread form which requires very careful alignment and again must be performed by a cam relieving action of the thread grinder. The entire process of producing the traditional cold-forming taps requires that only about 60-70 of such lobes can be ground per hour. Due to the mechanical linkages of the cam action, very careful work must be performed to form the lobes. Because of such mechanical linkages involved in the cam action of the thread grinder and the movements of the grinding wheel in relation to the tap itself, the production rate of actual taps can be as low as 20-25 taps per hour per operator.
Conventional cold-forming taps, when utilized, undergo tremendous stress so that any weakness in the tap causes immediate failure and since fluted taps are weaker than solid taps, they have a higher incidence of breakage although they are faster to use since the fluted taps allow room for lubrication while solid taps do not. Also because of the design of the fluted lobes of the tap, they must move a great deal of material across the forming surface as the tap rotates and thus a great deal of torque is required to drive the tap. A further problem that arises in the prior art is galling where the tap at some point welds to the material being tapped. The result can be tool breakage, oversized threads and poor finish which results occur in the prior art where there is a lack of lubrication, when the temperature of the workpiece has become elevated beyond a desired level or in situations where there is a large area of workpiece-tool engagement. Further, conventional cold-forming taps allow the workpiece material to close in around the tap giving a tight fit and under many conditions where tight gauging is necessary, serious problems can occur when production runs are made. In some situations it is necessary to gauge every piece and sort by hand to determine the tap size to be used in cases of contraction problems caused by using an over-sized conventional tap which stretches or expands the workpiece thereby allowing for the contraction of the workpiece material after the tap is removed. A further disadvantage of the prior art cold-forming taps is that when worn down, they cannot be reused into any other product.